On the evolutionary status and pulsations of the recently discovered Blue Large-Amplitude Pulsators (BLAPs)

TL;DR

This paper suggests that Blue Large-Amplitude Pulsators are hot pre-ELM white dwarf stars with pulsations driven by opacity mechanisms, providing insights into their evolutionary status and pulsation modes.

Contribution

It proposes a new evolutionary link for BLAPs as pre-ELM white dwarfs and models their pulsations using evolutionary sequences, explaining their observed variability.

Findings

BLAPs are well represented by pre-ELM WD models with ~0.34 Msun.

Observed pulsations are explained by high-order nonradial g-modes and low-order radial modes.

Pulsations are driven by the kappa mechanism at the Z-bump in opacity.

Abstract

The Blue Large-Amplitude Pulsators (BLAPs) constitute a new class of pulsating stars. They are hot stars with effective temperatures of ~30 000 K and surface gravities of log g ~ 4.9, that pulsate with periods in the range 20-40 min. Until now, their origin and evolutionary state, as well as the nature of their pulsations, were not been unveiled. In this paper, we propose that the BLAPs are the hot counterpart of the already known pulsating pre-Extremely Low Mass (pre-ELM) white dwarf (WD) stars, that are He-core low-mass stars resulting from interacting binary evolution. Using fully evolutionary sequences, we show that the BLAPs are well represented by pre-ELM WD models with high effective temperature and stellar masses ~ 0.34 Msun. From the analysis of their pulsational properties, we find that the observed variabilities can be explained by high-order nonradial g-mode pulsations or, in the case of the shortest periods, also by low-order radial modes, including the fundamental radial mode. The theoretical modes with periods in the observed range are unstable due to the kappa mechanism associated to the Z-bump in the opacity at log T ~ 5.25.